1. Field of the Invention
The present invention relates to a light intensity modulation or magnetic field modulation type magnetooptical recording apparatus and, more particularly, to improvements in drive devices for an electromagnet for generating erasing and recording bias magnetic fields in a light intensity modulation type apparatus, and for a magnetic head for generating a recording modulated magnetic field in a magnetic field modulation type apparatus.
2. Related Background Art
Conventionally, as typical magnetooptical recording methods for recording information on a magnetooptical recording medium, a light intensity modulation method and a magnetic field modulation method are known. The two recording methods will be briefly described below. FIG. 1 shows the schematic arrangement of a light intensity modulation type magnetooptical recording apparatus. Referring to FIG. 1, a recording layer 1a is formed on a magnetooptical disk 1 as an information recording medium. The disk 1 is rotated at a predetermined speed by a spindle motor 7. An electromagnet 2 as a magnetic field generator is arranged above the upper surface of the magnetooptical disk 1, and an optical head 5 is arranged below the lower surface of the disk 1 at a position opposite to the electromagnet 2. The electromagnet 2 is obtained by winding an excitation coil 2b around a magnetic core 2a, and generates a magnetic field for erasing or recording information when it is driven by an electromagnet drive circuit 3. The optical head 5 comprises a semiconductor laser 5a as a light source, an objective lens for converging a laser beam emitted from the laser 5a to a small beam spot, and radiating the beam spot on the recording layer 1a, and the like. The optical head 5 radiates a laser beam modulated according to an information signal onto the recording layer la when it is driven by a laser drive circuit 6.
When information is to be recorded, a control signal for instructing recording of information is supplied to the electromagnet drive circuit 3, and the electromagnet drive circuit 3 drives the electromagnet 2 on the basis of the instruction. Thus, a DC recording current is supplied to the excitation coil 2b of the electromagnet 2, and the electromagnet 2 generates a recording bias magnetic field in a predetermined direction and applies it to the magnetooptical disk 1. On the other hand, the laser drive circuit 6 generates a drive current modulated according to an input information signal, and drives the semiconductor laser 5a by the generated drive current. Thus, a laser beam from the semiconductor laser 5a is intensity-modulated in accordance with the information signal, and is radiated onto the recording layer 1a of the rotating magnetooptical disk 1. In this manner, the recording bias magnetic field and the modulated laser beam are applied to the recording layer 1a, and the direction of magnetization in a portion, irradiated with the laser beam, on the recording layer 1a aligns upward or downward in correspondence with the intensity of the laser beam. When the recording layer 1a is cooled upon rotation of the magnetooptical disk 1, the direction of magnetization is fixed, and an information mark corresponding to the information signal is recorded. Prior to this recording operation, an area to be recorded on the magnetooptical disk 1 is normally erased. When information is to be erased, the optical head 5 radiates a laser beam having a predetermined intensity onto the magnetooptical disk 1 while the electromagnet 2 applies an erasing bias magnetic field in a direction opposite to the recording bias magnetic field to the disk 1. Thus, the direction of magnetization of the recording layer 1 uniformly aligns, thereby erasing information.
FIG. 2 is a circuit diagram showing in detail the arrangement of the light intensity modulation type electromagnet drive circuit 3. Referring to FIG. 2, the excitation coil 2b of the electromagnet 2 is driven by transistors T1 to T4. FIG. 2 shows a bridge type drive circuit. The electromagnet drive circuit 3 includes a DC power supply V, a resistor R1 for setting the drive current of the excitation coil 2b to be a proper value, and an inverter 8 for inverting the information signal. The electromagnet drive circuit 3 receives a control signal from a controller (not shown), and drives the electromagnet 2 on the basis of the control signal. More specifically, when a low-level signal is input as the control signal, the transistors T1 and T4 are turned on, and the transistors T2 and T3 are turned off, thus supplying an erasing current to the excitation coil 2b in the direction of an arrow e in FIG. 2. On the other hand, when a high-level signal is input as the control signal, the transistors T1 and T4 are turned off, and the transistors T2 and T3 are turned on, thus supplying a recording current to the excitation coil 2b in the direction of an arrow w in FIG. 2. In this manner, in the electromagnet drive circuit 3, the electromagnet 2 is driven to generate an erasing or recording magnetic field by switching the direction of a current to be supplied to the excitation coil 2b in accordance with the control signal.
The magnetic field modulation method will be described below with reference to FIG. 3. Note that the same reference numerals in FIG. 3 denote the same parts as in FIG. 1, and a detailed description thereof will be omitted. Referring to FIG. 3, a magnetic head 9 generates a recording bias magnetic field. The magnetic head 9 is constituted by a magnetic core 9a, and an excitation coil 9b wound around the magnetic core 9a. When the direction of a current to be supplied to the excitation coil 9b is switched in correspondence with an information signal upon driving of a magnetic head drive circuit 10, the magnetic field generated by the magnetic head 9 is modulated in accordance with the information signal. On the other hand, a laser drive circuit 11 supplies a DC current to a semiconductor laser 5a, and the semiconductor laser 5a radiates a laser beam having a predetermined intensity onto a magnetooptical disk 1. Note that a control signal to be input to the laser drive circuit 11 is one for instructing switching of the laser beam from the semiconductor laser 5a between recording power and reproduction power in correspondence with recording and reproduction modes of information.
When information is to be recorded, the magnetic head drive circuit 10 supplies a drive current, which is modulated in accordance with an information signal, to the excitation coil 9b of the magnetic head 9. The magnetic head 9 generates a recording magnetic field modulated in accordance with the information signal, and applies the generated magnetic field to the magnetooptical disk 1. On the other hand, the semiconductor laser 5a in an optical head 5 receives a DC current from the laser drive circuit 11, and radiates a continuous laser beam onto the recording layer 1a. Upon radiation of the laser beam, the irradiated portion of the recording layer 1a is heated to a temperature equal to or higher than its Curie temperature, and the direction of magnetization in the irradiated portion of the recording layer 1a aligns in the direction of the bias magnetic field of the magnetic head 9. When the irradiated portion of the recording layer 1a is cooled upon rotation of the magnetooptical disk 1, the direction of magnetization in the irradiated portion of the recording layer 1a is fixed, and is recorded as an information mark in the direction of magnetization corresponding to the information signal. Note that the magnetic field modulation method does not require erasing prior to recording, unlike the light intensity modulation method, and new information can be overwritten on old information.
FIG. 4 is a circuit diagram showing an example of the magnetic head drive circuit 10 in the magnetic field modulation method. Referring to FIG. 4, the circuit 10 includes auxiliary coils L1 and L2, and transistors T5 and T6 used for switching the direction of a current to be supplied to the excitation coil 9b. The circuit 10 also includes a DC power supply V, a resistor R2 for setting the drive current of the excitation coil 9b to be a proper value, and an inverter 12 for inverting the information signal, and applying the inverted signal to the base terminal of the transistor T6. The drive current of the excitation coil 9b is modulated as follows in accordance with the information signal. When the information signal is at low level, the transistor T5 is turned off, and the transistor T6 is turned on, thus supplying a drive current to the excitation coil 9b in the direction of an arrow e in FIG. 4. When the information signal goes to high level, the transistor T6 is turned off, and the transistor T5 is turned on, thus supplying a drive current to the excitation coil 9b in the direction of an arrow w in FIG. 4. In this manner, the direction of the current to be supplied to the excitation coil 9b is switched in correspondence with the information signal, and the magnetic field generated by the magnetic head 9 is modulated according to the information signal. Note that since the inductances of the auxiliary coils L1 and L2 are sufficiently larger than that of the excitation coil 9b, the auxiliary coils L1 and L2 have constant current characteristics, thereby allowing high-speed inversion of the drive current of the excitation coil 9b.
In the electromagnetic drive circuit in the light intensity modulation method and the magnetic head drive circuit in the magnetic field modulation method, which have been described above with reference to FIGS. 2 and 4, a resistor or an element having a function equivalent to the resistor (e.g., a transistor) is arranged in the circuit so as to set the drive current of the electromagnet or the magnetic head to be a proper value. As for the resistor, for example, when a drive current having an amplitude I=0.4 A is to be supplied to the excitation coil 9b in the magnetic head drive circuit shown in FIG. 4, if the voltage of the DC power supply V is 5 V, and the sum total of the resistances r of the auxiliary coils, the excitation coil, and the transistors in the path of the drive current excluding the resistor R2 is r=5 .OMEGA., the resistor R2 is required to have a resistance of 7.5 .OMEGA.. However, when the resistor is arranged in the electromagnet drive circuit of the light intensity modulation method or in the magnetic head drive circuit of the magnetic field modulation method, as described above, the following two problems are posed.
As one problem, when the resistor (or an element having a function equivalent thereto) is arranged in the drive circuit, the power loss in the circuit becomes very large upon the supply of a drive current. For example, in the magnetic head drive circuit shown in FIG. 4, when the resistor R2 is set to have a resistance of 7.5 .OMEGA., and the drive current is set to be 0.4 A, as described above, the power loss across the resistor R2 becomes 1.2 W. The DC power supply V must supply a total electric power of 2 W including a power loss of 0.8 W consumed by other drive circuit elements. In this case, the power loss across the resistor R2 accounts for 60% of the power loss of the entire circuit. In recent years, a compact structure is required for an information recording/reproduction apparatus such as a magnetooptical recording/reproduction apparatus, and in order to satisfy such a requirement, high-density packaging of an electric circuit, and low power consumption must be realized. However, when the drive current is limited by a resistor or an equivalent element arranged in the drive circuit, the resistor or the like disturbs not only low power consumption of the drive circuit, but also high-density packaging due to heat generated by the element.
As the other problem, since the voltage of the DC power supply V of the drive circuit, and the resistance r of the drive current path in the drive circuit are not always constant, but vary depending on various factors such as a change in temperature, the drive current of the excitation coil in the electromagnet of the light intensity modulation method or in the magnetic head of the magnetic field modulation method varies. When the drive current of the electromagnet or the magnetic head varies, a bias magnetic field by the electromagnet or the magnetic head varies. For this reason, when the driven current varies in a direction to decrease, the bias magnetic field becomes too weak, thus causing a recording error.